U.S. Geological Survey and the California State Water Resources Control Board Groundwater Quality in the Western San Joaquin Valley, California Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s groundwater quality and increases public access to groundwater-quality information. The Western San Joaquin Valley is one of the study units being evaluated. The Western San Joaquin Valley Study Unit Overview of Water Quality The Western San Joaquin Valley (WSJV) study unit covers approximately Inorganic Organic 5,600 square miles and includes the Delta–Mendota and Westside subbasins of the constituents constituents San Joaquin Valley groundwater basin (California Department of Water Resources, 3 3 2003; Fram, 2017). The freshwater aquifer system in the WSJV study unit primarily is 17 composed of late Tertiary to Quaternary age alluvial and fluvial sediments derived from the Coast Ranges to the west and the Sierra Nevada to the east, and is divided into upper 53 30 94 and lower zones by a clay-rich lacustrine unit, the Corcoran Clay (Faunt, 2009). 121° 120° The groundwater resources used CONSTITUENT CONCENTRATIONS EXPLANATION er for public drinking water are defined High Moderate Low or not detected lumne Riv Corcoran Clay Tuo 37° extent as those parts of the aquifer system Pie charts illustrate the percentage of the primary 30' r Study area tapped by wells listed in the State of aquifer system, on an areal basis, with concentrations ve Ri boundary in the three specified categories. ced California’s database of public supply Mer Sierra Nevada wells. In the Delta–Mendota subbasin, GAMA’s Priority Basin Project evalu- S public supply wells are typically drilled an D ates the quality of untreated groundwater. el ta DELTA– J to depths of 150 to 600 feet, consist San o a er For context, however, concentrations 37° Luis MENDOTA q no Riv M u Fres of solid casing from land surface to i Reservoir e n measured in groundwater are compared to n do ta C an depths of 130 to 290 feet, and are a benchmarks established for drinking-water l ver Ri Coast Ranges screened below the solid casing. In the quality, such as maximum contaminant C Westside subbasin, public supply wells a levels (MCL). A concentration above a li fo are typically drilled to depths of 575 benchmark is defined as high. Benchmarks rn Fr ia es no 36° S to 1,550 feet, and have solid casing and definitions of moderate and low con- l o u 30' WESTSIDE centrations are discussed in the inset box on g from land surface to depths of 375 h to 650 feet. Zones above and below page 3. Aq ue d Many inorganic constituents exist u the Corcoran Clay are used for public c t drinking water in both subbasins. Water naturally in groundwater, and their concen- 0 10 20 MILES trations can be affected by natural processes quality in groundwater resources used 0 10 20 KILOMETERS as well as by human activities. In the WSJV 36° for public drinking water can differ study unit, one or more inorganic constitu- Base modified from U.S. Geological Survey and other Federal and from that in shallower and deeper parts State digital data, various scales; Albers Equal Area Projection; ents were present at high concentrations North American Datum of 1983 of the aquifer system. in about 53 percent of the groundwater resources used for public drinking water. The hydrology of the WSJV study unit has been extensively altered by human Organic constituents are found in activity (Faunt, 2009). Under natural conditions, the primary sources of groundwater products used in the home, business, rechargesac16-0602_fig were infiltration 03 of sparse streamflow from the Coast Ranges through alluvial industry, and agriculture, and can enter fans and from the San Joaquin and Kings Rivers in the basin, primarily during the win- the environment through normal usage, ter rainy season. Agricultural development began more than a century ago, and about spills, or improper disposal. Some organic constituents also can have natural sources, 70 percent of the land now is used for agriculture. Irrigation with groundwater and such as petroleum deposits. In this study imported surface water is now the primary source of groundwater recharge, and pump- unit, organic constituents were present at ing for agricultural use is now the primary source of discharge from the aquifer system. high concentrations in about 3 percent of Drainage from WSJV agricultural lands can have high concentrations of trace elements the groundwater resources used for public and salts and, thus, must be managed. drinking water. U.S. Department of the Interior ISSN 2327-6916 (print) Fact Sheet 2017–3028 U.S. Geological Survey ISSN 2327-6932 (online) April 2017 https://doi.org/10.3133/fs20173028 sac16-0602_fig pies RESULTS: Groundwater Quality in the Western San Joaquin Valley INORGANIC CONSTITUENTS Inorganic Constituents with Human-Health Benchmarks Trace elements are naturally present in the minerals of rocks and sediments 21 and in the water that comes into contact with those materials. In the WSJV study Trace unit, one or more trace elements were present at high concentrations in about 52 27 elements 52 percent of the groundwater resources used for public drinking water. Six trace elements were present at concentrations above benchmarks: boron (51 percent), hexavalent chromium (25 percent), arsenic (10 percent), strontium (5 percent), molybdenum (4 percent), and selenium (3 percent). Different combinations of trace elements are present at high concentrations in different areas of the WSJV 3 5 study unit because the compositions of the rocks and sediments through which Uranium and groundwater recharge passes and the geochemical conditions in the groundwater radioactive that control the solubility of minerals vary across the study unit. 92 Radioactivity is the release of energy or energetic particles during sponta- constituents neous decay of unstable atoms. Most of the radioactivity in groundwater comes from the decay of natural isotopes of uranium and thorium in minerals in aquifer materials. Radioactive constituents were at high levels in about 3 percent of the groundwater resources used for public drinking water. Uranium, radium, and 4 gross alpha-particle activity were the radioactive constituents present at high 15 levels. Nutrients, including nitrate, are naturally present at low concentrations in Nitrates groundwater, and high concentrations generally are caused by human activities. 81 Common sources of nutrients include fertilizer applied to crops and landscaping, seepage from septic systems, and human and animal waste. Nitrate was present at high concentrations in about 4 percent of the groundwater resources used for public drinking water. Inorganic Constituents with Non-Health Benchmarks 6 (Not included in water-quality overview charts shown on the front page) Total Some constituents affect the aesthetic properties of water, such as taste, 44 50 dissolved color, and odor, or can create nuisance problems, such as staining and scaling. solids The benchmarks used for these constituents were non-regulatory secondary maximum contaminant levels. Total dissolved solids (TDS) concentration is a measure of the salinity of the groundwater, and all natural waters contain TDS as a result of the weathering and dissolution of minerals in rocks and sediments. The State of California has a rec- ommended and an upper limit for TDS in drinking water. Concentrations of TDS 46 46 Manganese were high (greater than the upper limit) in about 50 percent of the groundwater or iron resources used for public drinking water. Anoxic conditions in groundwater (low amounts of dissolved oxygen) can 8 result in release of manganese, iron, and other associated trace elements from minerals into groundwater. Manganese or iron was present at high concentrations in about 46 percent of the groundwater resources used for public drinking water. SPECIAL-INTEREST CONSTITUENT Perchlorate (Not included in water-quality overview charts shown on the front page) 15 Perchlorate is an inorganic constituent that has been regulated in California Perchlorate drinking water since 2007. It is an ingredient in rocket fuel, fireworks, safety flares, and some fertilizers, and also exists naturally at low concentrations in 85 groundwater. In the WSJV study unit, perchlorate was not detected at high con- centrations in the groundwater resources used for public drinking water and was detected at moderate concentrations in about 15 percent. sac16-0602_fig pies RESULTS: Groundwater Quality in the Western San Joaquin Valley ORGANIC CONSTITUENTS 1 Organic Constituents with Human-Health Benchmarks 3 The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human- VOCs health benchmarks. Pesticides and VOCs detected at these very low concentrations can be used to help trace water movement from the landscape into the aquifer system. 96 Many household, commercial, industrial, and agricultural products contain VOCs, which are characterized by their tendency to volatilize (evaporate) into the air. In the WSJV study unit, VOCs were present at high concentrations in about 3 percent of the groundwater resources used for public drinking water. Benzene and tetrachloroethene (PCE) were the VOCs found at high concentrations. The likely source of the benzene was nearby natural petroleum deposits (Fram, 2017). PCE is a commonly used solvent. 3 Pesticides, including herbicides, insecticides, and fumigants, are applied to crops, gardens, lawns, around buildings, and along roads to help control unwanted vegeta- Pesticides tion (weeds), insects, fungi, and other pests. In the WSJV study unit, pesticides were not found at high concentrations in the groundwater resources used for public drinking 97 water. The pesticide found at moderate concentrations was the fumigant 1,2-dibromo- 3-chloropropane (DBCP).